Monday, January 13, 2014

Intermolecular forces and melting/boiling point

One thing that tends to make confusion
to my students is the relationship between non-covalent intermolecular forces
and the melting/boiling point of a substance. In fact, how one can explain in a
molecular point of view, the existence of the liquid and solid states? The
answer lies precisely by the existence of intermolecular forces. Both states
are characterized by a higher order spatial distribution of the molecules
compared to the gaseous state. This order is a consequence of the attractions between
the different molecules that make up a substance. Thus, in the liquid state,
each molecule establishes various interactions (non-covalent!) with neighboring
molecules, and in the solid state the amount of interactions is further
increased.

When it is reached the
melting/boiling point, which is going on is a transition from a state in which
there are more intermolecular forces to one where there are fewer. In other
words, what is being done is to provide energy to break the non-covalent forces
established between the molecules, by increasing their kinetic energy.

Therefore, the greater the
intensity of the non-covalent forces existing between the molecules, the
greater the amount of energy required to break these interactions, and thus the
higher the melting/boiling point.

I often ask to my students
what happens when it is reached the boiling point of a substance, and the
answer that I invariably get is: "You are breaking the bonds." The
problem comes when I ask what kind of bonds, because usually their tendency is
to say the covalent bonds. I, playfully, then tell them that if so, the water
in the gaseous state is no longer H2O, and then they realize that indeed
that what it is cleaved are not the covalent bonds but the non-covalent ones.

In short, there is a direct
relationship between intermolecular forces and melting/boiling point, and the greater
the sum of these forces, the higher the melting/boiling point of a substance.